One, Most likely those ads are in pay-to-post categories, so each one of those ads is providing CL the revenue it needs to pay its expenses to keep the rest of the site free to use
Two, CL provides quite powerful tools, some of which are very trivial to use, to fine-tune what ads you find when searching, and to avoid the vast majority of unwanted ads.
On the latter, here are two of the simplest things you can do:
ONE - be sure to first select the most appropriate specific category before searching. Don't search "all for sale"
TWO - if you want to avoid all paid "dealer" ads, select to see only "by owner" ads
There are much more powerful tools. To get assistance on how to use those, post to the HELP DESK forum, and be sure to include all of these specific details:
- which city/site(s) you are searching
- what specifically you are searching for, and what words you are using to do so
- some idea as to the specific unwanted/duplicate ads you're getting.
The most effective way to show the first two items is to include an actual search URL, like this one:
Which is a search of the "for-sale, furniture" category for "fresser" on the Nashville site
Note that your forums handle might be too new to post links and you might get an error if you try to post on - if that happens post the URL like this, with the "https://" part left off: